Historically, experts believed that patients with healing bones of the lower extremities (following surgery, reduction, or replacement) should refrain from placing any compressive force on the extremity, as happens when walking, until the bone had begun to heal. Today, orthopedists and researchers have learned that placing some force on the limb will generate piezoelectric currents which stimulate bone healing. Excessive force, however, may result in delayed union, nonunion or possibly malunion of the bone. Thus for each fracture type (spiral, oblique, transverse and so on) and location of fracture an optimal and measurable range of force should be placed across the fracture to maximize healing potential. The peak force needs to be monitored in each gait cycle to be certain that it does not exceed a maximum limit prescribed by the physician. Total force translated through the heel should be measured regardless of whether the patient first strikes the heel or the toe.
When physicians provide written orders to physical therapists regarding ambulation training, they typically instruct "partial weight bearing status". These instruction are vague with the result that some patients may place inadequate force on the limb while others apply excessive force on the limb. Healing may be delayed when inadequate force is applied. When excessive force is transmitted the healing properties may also be adversely affected resulting in further disability and the need for extended and costly care. Additional surgeries and lengthy immobilization, along with their associated risks, represent a few of the potential complications. With the development of porous coated joint replacements, the need to monitor weight bearing is also critical to optimize the potential for bone ingrowth into the porous interface.
A device to warn of excessive force applied to a lower extremity is described in U.S. Pat. No. 3,791,375 to Pfeiffer. That patent shows two sensors for positioning beneath a patient's foot connected to a sensing/signal unit to be secured to the patient's ankle. Each sensor is comprised of two plates spaced apart by a fluid. The sensor requires a resilient spacer to keep the two plates apart at zero force, but no specific materials are suggested. Upon application of a force, the resilient spacer is deformed allowing the two plates to come closer together displacing some of the fluid between them. The fluid displaced travels to bellows within the signal unit which expand. When the bellows expand enough, a switch arm is engaged and pushed into its opposing contact. An audible alarm sounds when the electrical circuit is completed. The patent does not specify any particular type of audible alarm. The patent describes adjustment of the space between the switch arm and the opposing contact so that the force required to bring the two parts together can be varied. However, this method of establishing and varying the set limit is imprecise as the resistance of the spring would be expected to decrease after extended use. Calibration against an external reference is required for precise setting of the force limit. Once the set limit has been set repeated calibration and adjustment would be required to maintain the desired limit. Of necessity the sensor and sensing/signal unit are connected. Sensors that measure force by detecting deformation of a resilient spacer suffer from the need for costly, complex mechanisms which in use over a lengthy period would be expected to break.
Pfeiffer also describes an electro-mechanical embodiment which like the hydraulic embodiment dependents on variation in the distance between two deformable plates as an indication of force. In that embodiment the top plate of the sensor has a contact mounted on its lower surface. The bottom plate has a switch arm mounted on its upper surface. The distance between the switch arm and the contact at zero force is set with a set screw. Upon deformation of the upper plate the contact and switch arm meet to complete an electric circuit which actuates the signal.
A resistance sensor offered by Tekscan, Inc. (Boston, Mass.) is described in SENSORS, pp 21-25, May 1991. This monitor is a force mapping system for gait analysis. The force sensor comprises a grid of conductive ink. The rows are separated from the columns by a coat of a force sensitive resistive ink having resistance inversely proportional to force. The force distribution over the sensor's surface is determined by scanning the grid and measuring resistance at each intersection. The system includes a signal detecting/transmission unit designed as an ankle pack having hardware to drive and multiplex the circuit, amplifiers and multiplexors to receive and transmit the output current, analog to digital converter and a parallel to serial converter. The analog to digital convertor in the signal detection unit requires substantial volume, uses significant power and is costly. Finally, the system includes a signal measurement/analysis circuit where the data stream from the ankle pack is reconverted to analog and analyzed. The system's software provides real-time two and three dimensional views of force patterns applied to the sensor. For calibrated measurements the foot pad must be calibrated in a calibration frame.
A capacitance sensor under development at Case Western Reserve University to measure grasp force of a hand is briefly described in SENSORS, p. ,19.sub.--. This sensor comprises chromium and gold rows and columns pattern coated onto polyimide substrates. The pattern coated films are separated by a layer of silicone which forms a compliant dielectric layer. The circuitry to detect, transmit and analyze and report the force applied to the sensor are not described.
A need exists, therefore, for a simple effective monitor to determine if a force applied by a patient to a limb exceeds a value set for the patient and to warn the patient when the predetermined upper limit is exceeded. Today no such device exists.